Since a vehicle headlight requires the precise control of light distribution, unlike a general illumination lamp, the vehicle headlight needs to be uniformly formed to have a rod shape and the light emitting form needs to have a high contrast ratio. Since a filament of an incandescent lamp or a halogen lamp has these characteristics, a halogen lamp or an incandescent lamp is widely used as a light source for a vehicle headlight.
Meanwhile, the light intensity of a discharge lamp is higher than that of an incandescent lamp or a halogen lamp. Accordingly, in a vehicle headlight where a discharge lamp is used as a light source, it may be possible to improve the brightness and to obtain a longer life of a vehicle headlight using a discharge lamp is longer than that of a vehicle headlight using each of an incandescent lamp and a halogen lamp.
Since the brightness of a discharge lamp is higher than that of an incandescent lamp or a halogen lamp and the life of a discharge lamp is longer than that of an incandescent lamp or a halogen lamp as described above, use of a discharge lamp as a vehicle headlight has spread in recent years.
In general, a discharge lamp has a light emitting tube which holds a pair of electrodes and which holds a gas, such as inert gas, is sealed in an outer tube that protects the light emitting tube or stabilizes the temperature of the light emitting tube. The light emitting tube includes a light emitting portion in which discharge occurs and a pair of fine tube portions that are provided on the opposite sides of the light emitting portion. The light emitting portion is a portion where an arc is generated when discharge occurs, and the diameter of the light emitting portion is larger than that of the fine tube portion.
When a high-voltage pulse is applied to an electrode, discharge occurs in the light emitting portion of the light emitting tube, so that the discharge lamp starts to light up.
As examples of a lighting system of the discharge lamp, there are both AC lighting systems and DC lighting systems (see JP-A-2007-250225, for instance). In the discharge lamp using a DC lighting system, a pair of electrodes are referred to as a cathode-side electrode and an anode-side electrode, respectively.
Since the polarities of the electrodes are not switched in the discharge lamp using a DC lighting system, the discharge lamp using a DC lighting system has an advantage of reducing the loads of the electrodes as compared to a discharge lamp using an AC lighting system where a cathode-side electrode and an anode-side electrode can be exclusively designed for the loads of the electrodes, respectively.
Meanwhile, it can be important that a vehicular discharge lamp used as a light source for a vehicle light has reliable start-up (starting performance of 100%) and a quick rising characteristic after the start-up as compared to a general illumination discharge lamp. Accordingly, in the vehicular discharge lamp, in order to accelerate the rising characteristic, the power immediately after lighting (starting) is increased and the pressure of the inert gas to be sealed is increased.
Meanwhile, if the pressure of the inert gas is increased in order to accelerate the rising characteristic, the starting voltage is increased at the time of the start-up of the vehicular discharge lamp. For this reason, the reliability of the start-up may be impaired.
Further, the size of a starting circuit which applies a starting voltage to the discharge lamp may be increased in order to increase the starting voltage, leading to increased manufacturing costs.